1. Field of the Invention
This invention relates to a process for the production of purified propylene homopolymers or copolymers in industrially advantageous manner.
2. Description of the Prior Art
Homopolymerization or copolymerization of propylene in liquid propylene (hereinafter referred to as "bulk polymerization") is known and widely applied in industry.
In comparison with the polymer slurry produced using conventional suspension (or solvent) polymerization (referred to as "conventional solvent polymerization" hereinafter) in which the polymerization is generally carried out in a liquid saturated hydrocarbon solvent having 5 or more, preferably 5 to 7, carbon atoms, the polymer slurry produced using bulk polymerization is quite advantageous in that separation of the polymer produced from the solvent can be carried out very easily by merely subjecting the slurry to reduced pressure because the unsaturated hydrocarbon monomers (mainly propylene) used as a polymerization solvent have such a high vapor pressure that they are easily vaporized.
In general, however, polymers obtained by this bulk polymerization process have some disadvantages because they contain large amounts of both polymers soluble in boiling n-heptane as a by-product and catalyst residues. Accordingly, such polymers have inferior physical properties, as described hereinafter.
As to polymers soluble in boiling n-heptane, when bulk polymerization is carried out, for example, with a combined catalyst of titanium trichloride and triethyl aluminum, the produced polymer contains only 75 to 85% by weight of boiling n-heptane-insoluble polymer. Also, when a combined catalyst of titanium trichloride and diethyl aluminum chloride is used, the content of boiling n-heptane-insoluble polymer is 85 to 95% by weight.
Such polymers containing large amounts of boiling n-heptane-soluble polymers have inferior physical properties, particularly with respect to stiffness and tackiness. Therefore, these polymers can be used for molding but they are not suitable for films and fibers requiring relatively good physical properties.
As to the catalyst residues, on the other hand, a reduction in the amount of the catalyst residue in bulk polymerization is inherently possible, since the polymerization has a high polymerization rate and can be operated at a high level of catalyst efficiency as compared with conventional solvent polymerization. However, a very large amount of organoaluminum compounds must be used in order to maintain high levels of catalyst efficiency and selectivity. For example, when bulk polymerization of propylene is carried out in liquid propylene using highly active titanium trichloride and diethyl aluminum chloride, as is disclosed in Japanese Patent Application (OPI) No. 34478/72 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), high levels of activity and selectivity cannot be maintained unless 0.2 g or more of diethyl aluminum chloride per kg of liquid propylene is used. When large amounts of the organoaluminum compound remain in the polymer as a catalyst residue, the produced polymers have markedly reduced stability, and are easily degraded by heat, thus resulting in foaming and coloration. Consequently, such polymers cannot be used when high quality is desired.
In order to obtain high quality propylene polymers, therefore, the boiling n-heptane-soluble polymers and catalyst components remaining as residue must be removed.
Extensive investigations were made to improve the quality of such polymers since for a polymer slurry produced using bulk polymerization, polymers soluble in the slurry, residual catalyst dissolved in the slurry and a part of residual catalyst deposited from the slurry can easily be separated from the slurry in a counter-current washing tower using liquid propylene as a washing solvent. This led to an industrially advantageous process for producing homopolymers and copolymers of propylene (i.e., as described in Japanese Patent Application (OPI) Nos. 79589/75, 139886/76 and 3679/77 and U.S. Pat. No. 4,053,697).
According to this process, the catalyst residue resulting from the organoaluminum compound can greatly be removed. However, the catalyst residue resulting from the titanium trichloride solid catalyst is not substantially removed and remains and, furthermore, polymers having high molecular weights are easily formed in the washing tower. Therefore, the polymers obtained by the above process, when molded at high temperatures, provide those films having low levels of hue which are inferior in thermal stability. Moreover, when they are neutralized by adding fatty acid metal salts, etc. (because when left as are, they corrode metals) a freed fatty acid adheres onto metal rolls during the production of films and makes the metal rolls dirty. In addition, the high molecular weight polymers formed in the washing tower markedly deteriorate the transparency of films. Thus, they cannot be used as are in the manufacture of products for which high quality is required.
An improved method, therefore, has been proposed in which alcohols containing 3 to 8 carbon atoms or mixtures of such an alcohol and an epoxide, the molar amounts of the alcohol and epoxide to the organoaluminum compound being, respectively, 0.1 to 10 and 0.1 to 3, are introduced into the washing tower (see U.S. Pat. No. 4,182,852). According to this method, the formation of high molecular weight polymers in the washing tower can be prevented, but a large amount of the catalyst residue resulting from the titanium trichloride solid catalyst still remains without removal.
According to the examples of U.S. Pat. No. 4,182,852, although the pellet color of the polymers is good, TiO.sub.2 remains in an amount of 14 to 39 ppm, and when molded at high temperatures, films having low levels of hue are obtained. Therefore, as proposed in Japanese Patent Application (OPI) No. 68889/79, it has been required to add an additional step in which the polymer slurry is further brought in contact with gaseous epoxides.
A principal object of this invention, therefore, is to provide an improved method for industrially advantageously producing propylene homopolymers or copolymers which are much more purified as compared with those obtained by the above-described prior art methods, without providing any additional step.
As a method of purifying polyolefins obtained by bulk polymerization, Japanese Patent Publication No. 29946/75 proposes a treatment with an epoxide in an amount of 15 mols or more per mol of the catalyst component and subsequent washing with a lower hydrocarbon. Additionally, Japanese Patent Publication No. 1274/76 proposes a treatment with an alcohol containing 3 or more carbon atoms and an alkylene oxide and subsequent washing with a lower hydrocarbon.
These methods, however, need complicated treating and washing steps, and they cannot be said to be a simplified method. Furthermore, they need a relatively large amount of a treating agent, and they are thus not advantageous from the industrial standpoint.